Ferrite magnetic material and method for producing hexagonal w type ferrite magnetic material

ABSTRACT

The present invention provides a ferrite magnetic material comprising as a main constituent a compound represented by a composition formula, AFe 2+   a Fe 3+   b O 27  (wherein 1.1≦a≦2.4, 12.3≦b≦16.1; and A comprises at least one element selected from Sr, Ba and Pb), and also comprising as additives a Ca constituent in terms of CaCO 3  and a Si constituent in terms of SiO 2  so as to satisfy the relation CaCO 3 /SiO 2 =0.5 to 1.38 (molar ratio). By making the relation CaCO 3 /SiO 2 =0.5 to 1.38 (molar ratio) be satisfied, the coercive force (HcJ) and the residual magnetic flux density (Br) can be made to simultaneously attain high levels.

TECHNICAL FIELD

The present invention relates to a hard ferrite material, in particular,a ferrite magnetic material suitably usable for a hexagonal W-typeferrite magnet.

BACKGROUND ART

Magnetoplumbite-type hexagonal ferrites typified by SrO.6Fe₂O₃, namely,M-type ferrites have hitherto been mainly used for sintered magnets. Asfor such M-type ferrite magnets, attempts have been made to attain highperformances by focusing on making the ferrite grain sizes approach thesingle-domain grain sizes, aligning the ferrite grains along themagnetic anisotropy directions and attaining high densities. As a resultof such attempts, the properties of the M-type ferrite magnets areapproaching the upper limits thereof to lead to a situation such thatfurther drastically improved magnetic properties of the magnetsconcerned are hardly to be desired.

W-type ferrite magnets are known as such ferrite magnets that have apossibility of exhibiting magnetic properties superior to those of theM-type ferrite magnets. The W-type ferrite magnets are higher by about10% in saturation magnetization (4πIs) than the M-type ferrite magnetsand comparable in anisotropy field with the M-type ferrite magnets.Patent Document 1 (National Publication of International PatentApplication No. 2000-501893) discloses a W-type ferrite magnet having acomposition represented by SrO.2(FeO).n(Fe₂O₃) with n satisfying7.2≦n≦7.7, having a sintered body mean grain size of 2 μm or less and a(BH)max value of 5 MGOe or more, and describes that the W-type ferritemagnet is produced through the steps of (1) mixing SrCO₃ and Fe₂O₃ witheach other in a required molar ratio, (2) adding C to the raw materialpowder, (3) calcining, (4) separately adding CaO, SiO₂ and C aftercalcining, (5) milling to a mean particle size of 0.06 μm or less, (6)compacting the obtained milled power in a magnetic field, and (7)sintering in a nonoxidative atmosphere.

Patent Document 2 (Japanese Patent Laid-Open No. 11-251127) discloses,as a W-type ferrite magnet having a maximum energy product exceedingthose of conventional M-type ferrites and having a composition differentfrom the conventional ones, a ferrite magnet characterized in that thebasic composition thereof is represented in terms of atomic ratio byMO.xFeO.(y−x/2)Fe₂O₃ (M comprises one or more of Ba, Sr, Pb and La) withthe proviso that 1.7≦x≦2.1 and 8.8≦y≦9.3.

Patent Document 3 (Japanese Patent Laid-Open No. 2001-85210) discloses,as a ferrite sintered magnet having magnetic properties superior tothose of conventional M-type ferrites, a ferrite sintered magnetcomposed of a composite material in which a W-type ferrite phaserepresented by a composition formula AO.2(BO).8Fe₂O₃, wherein Acomprises one or more of Ba, Sr, Ca and Pb, and B comprises one or moreof Fe, Co, Ni, Mn, Mg, Cr, Cu and Zn, and the W-type ferrite phasecoexists with one or two of an M-type ferrite phase represented by acomposition formula AO.6Fe₂O₃, wherein A comprises one or more of Ba,Sr, Ca and Pb, and a magnetite phase represented by a compositionformula Fe₃O₄, the ferrite sintered magnet being characterized by havinga molar ratio of the W-type ferrite phase ranging from 60 to 97%, ameangrain size thereof ranging from 0.3 to 4 μm and a magnetic anisotropyrelated to a particular direction.

The W-type ferrite magnets disclosed in Patent Documents 1 to 3 havebeen investigated by focusing on the basic composition (the maincomposition) in such a way that, for example, in Patent Document 1, n inSrO.2(FeO).n(Fe₂O₃) is set to fall within a range from 7.2 to 7.7. Onthe other hand, in ferrite magnets, predetermined amounts of SiO₂ andCaCO₃ are added as additives for the purpose of improving the coerciveforce or regulating the grain size. Accordingly, it is important toinvestigate the main composition by also taking the additives intoconsideration for the purpose of obtaining practical W-type ferritesintered magnets. However, such investigations have not been reportedalso in Patent Documents 1 to 3.

Accordingly, the present invention takes it as an object to provide anoptimal composition of a ferrite magnetic material wherein the optimalcomposition takes even additives into consideration.

DISCLOSURE OF THE INVENTION

The present invention has been achieved in view of such technicalproblems as described above, and provides a ferrite magnetic materialcharacterized by comprising as a main constituent a compound representedby a composition formula, AFe²⁺ _(a)Fe³⁺ _(b)O₂₇ (wherein 1.1≦a≦2.4,12.3≦b≦16.1; and A comprises at least one element selected from Sr, Baand Pb), and also by comprising as additives a Ca constituent in termsof CaCO₃ and a Si constituent in terms of SiO₂ so as to satisfy therelation CaCO₃/SiO₂=0.5 to 1.38 (molar ratio).

The ferrite magnetic material of the present invention makes it possibleto simultaneously attain a coercive force (HcJ) of 3 kOe or more and aresidual magnetic flux density (Br) of 4.5 kG or more by optimizing thecompositions of the main constituent and the additives and further byimproving the production steps.

The ferrite magnetic material of the present invention can bepractically used in a variety of forms.

Specifically, the ferrite magnetic material according to the presentinvention can be applied to ferrite sintered magnets. When applied toferrite sintered magnets, it is preferable that the sintered bodiesconcerned each have a mean grain size of 0.8 μm or less.

The ferrite magnetic material according to the present invention canalso be applied to ferrite magnet powders. Such ferrite magnet powderscan be used for bonded magnets. In other words, the ferrite magneticmaterial according to the present invention can constitute bondedmagnets as ferrite magnet powders to be dispersed in resins.

The ferrite magnetic material according to the present invention canalso constitute magnetic recording media as film-like magnetic layers

The ferrite magnetic material according to the present inventionpreferably has as its main phase a hexagonal W-type ferrite (a W phase).The main phase as referred to herein means that the molar ratio of the Wphase as derived from the X-ray diffraction intensity amounts to 70% ormore. According to the ferrite magnetic material of the presentinvention, it is possible to make the W phase be a single phase, inother words, to make the molar ratio of the W phase almost equal to100%.

The ferrite magnetic material according to the present inventionpreferably comprises a Ca constituent in terms of CaCO₃ in an amountwithin a range from 0.3 to 1.5 wt % and a Si constituent in terms ofSiO₂ in an amount within a range from 0.1 to 1.8 wt %.

The present invention also provides a method for producing a hexagonalW-type ferrite magnetic material comprising the steps of: (a) obtaininga raw material powder comprising A (wherein A comprises at least oneelement selected from Sr, Ba and Pb) and Fe; (b) obtaining a calcinedbody by maintaining the raw material powder at a predeterminedtemperature for a predetermined time; and (c) milling the calcined body,wherein CaCO₃ and/or SiO₂ are added before and/or after said step (b) sothat the hexagonal W-type ferrite magnetic material comprises a Caconstituent in terms of CaCO₃ and a Si constituent in terms of SiO₂ soas to satisfy the relation of molar ratio CaCO₃/SiO₂=0.5 to 1.38.

The above described production method comprises the followingembodiments.

In one embodiment, when the Ca constituent in terms of CaCO₃ and the Siconstituent in terms of SiO₂ are added before the step (b) in a molarratio of CaCO₃/SiO₂=0.5 to 1.38, the ferrite magnet powder is preparedby milling in the step (c). Also, in another embodiment, when CaCO₃ andSiO₂ are added similarly before the step (b) in a molar ratio ofCaCO₃/SiO₂=0.5 to 1.38, a ferrite sintered magnet is prepared bysintering the milled powder obtained in the step (c).

Further, in one embodiment, when the Ca constituent in terms of CaCO₃and the Si constituent in terms of SiO₂ are added by adding CaCO₃ andSiO₂ after the step (b) in a molar ratio of CaCO₃/SiO₂=0.5 to 1.38, aferrite sintered magnet is prepared by sintering the milled powderobtained in the step (c). Also, in another embodiment, the ferritemagnet powder is prepared by milling the ferrite sintered magnet.

In still another embodiment in the present invention, when the Caconstituent in terms of CaCO₃ and the Si constituent given in terms ofSiO₂ are provided in such a way that only CaCO₃ is added before the step(b) and CaCO₃ and/or SiO₂ are added after the step (b) so as for themolar ratio between CaCO₃ and SiO₂ to satisfy the relationCaCO₃/SiO₂=0.5 to 1.38, the ferrite sintered magnet is prepared bysintering the milled powder obtained in the step (c).

In the method for producing the hexagonal W-type ferrite magneticmaterial of the present invention, the raw material powder comprisesACO₃ and Fe₂O₃ preferably in a molar ratio of 1:8.0 to 1:8.6, and morepreferably in a molar ratio of 1:8.3 to 1:8.5.

When the ferrite sintered magnet of the present invention is produced,all of a predetermined amount of A (at least one element selected fromSr, Ba and Pb) can be added before the step (b), or alternatively, afraction of the predetermined amount of A can be added after the step(b). By adopting such production steps, an improvement of the magneticproperties can be attained. In other words, in the present invention,the raw material powder comprises an ACO₃ powder and a Fe₂O₃ powder, andthe step (c) for milling the calcined body can be carried out after apredetermined amount of the ACO₃ powder is added after the step (b) forobtaining the calcined body.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart showing the outline of the steps for production ofa ferrite sintered magnet according to the present invention;

FIG. 2 is a table showing the compositions, magnetic properties andstructures of the magnetic materials in Example 1;

FIG. 3 is a table showing the relation between CaCO₃/SiO₂ and the meangrain size of the sintered magnet in Example 1;

FIG. 4 is a table showing the measurement results of the magneticproperties when SrCO₃ is added after calcining in Example 1;

FIG. 5 is a table showing the compositions, magnetic properties andstructures of the magnetic materials in Example 4;

FIG. 6 is a table showing the measurement results of the mixingcomposition, the magnetic properties of sintered bodies and the like inExamples 4 to 7;

FIG. 7 is a graph showing the relation between the additive amount of aCa constituent at the time of mixing and the coercive force (HcJ) inExample 4;

FIG. 8 is a graph showing the relation between the additive amount ofthe Ca constituent at the time of mixing and the residual magnetic fluxdensity (Br) in Example 4;

FIG. 9 is a graph showing the relation between the additive amount ofthe Ca constituent at the time of mixing and the mean grain size inExample 5; and

FIG. 10 is a graph showing the relation between the additive amount ofthe Ca constituent at the time of mixing and the coercive force (HcJ) inExample 5.

BEST MODE FOR CARRYING OUT THE INVENTION

In the following, the present invention will be described in detail withreference to the embodiments thereof.

The W-type ferrites include a Zn—W-type ferrite and a Fe—W-type ferrite.The Zn—W-type ferrite containing Zn in the composition thereof exhibitsa higher residual magnetic flux density (Br) than the Fe—W-type ferrite.The Zn—W-type ferrite also has an advantage that it is easily compatiblewith mass production because it can be sintered in the air. On the otherhand, the Zn—W-type ferrite has a drawback that the coercive force (HcJ)thereof is low because the anisotropy field thereof is low. For thepurpose of obtaining a high performance W-type ferrite by solving thisproblem, the Fe—W-type ferrite containing Fe²⁺ in the compositionthereof is taken as a target of the present invention.

In the present invention, when the molar ratio of the W phase is 70% ormore, the W phase is referred to as the main phase. From the viewpointof the magnetic properties, the molar ratio of the W phase is desirably70% ormore, preferably 95% or more, and more preferably almost 100% (asingle phase). The molar ratio in the present invention is derived asfollows: a standard sample is prepared by mixing the powders of a W-typeferrite, an M-type ferrite, hematite and spinel in a predetermined ratiotherebetween; the X-ray diffraction intensities of the standard samplethus prepared are measured in advance, and the molar ratio is derivedfrom a comparison with the X-ray diffraction intensities thus obtainedas a standard (this is also the case for Examples to be describedlater).

The ferrite magnetic material of the present invention has a maincomposition represented by the following composition formula (1):AFe²⁺ _(a)Fe³⁺ _(b)O₂₇   (1)wherein 1.1≦a≦2.4; 12.3≦b≦16.1; and A comprises at least one elementsselected from Sr, Ba and Pb. As A, at least one of Sr and Ba ispreferable, and Sr is particularly preferable from the viewpoint of themagnetic properties.

The variable a representing the proportion of Fe²⁺ is set to fall in therange 1.1≦a≦2.4. When a is less than 1.1, the M phase and the Fe₂O₃(hematite) phase, both lower in saturation magnetization (4πIs) than theW phase, are generated to degrade the saturation magnetization (4πIs).On the other hand, when a exceeds 2.4, the spinel phase is generated todegrade the coercive force (HcJ). Accordingly, a is set to fall in therange 1.1≦a≦2.4. The range of a is preferably 1.5≦a≦2.4 and morepreferably 1.6≦a≦2.1.

The variable b representing the proportion of Fe³⁺ is set to fall in therange 12.3≦b≦16.1. When b is less than 12.3 or exceeds 16.1, high levelsof the coercive force (HcJ) and the residual magnetic flux density (Br)cannot be attained simultaneously. The range of b is preferably12.9≦b≦15.6 and more preferably 12.9≦b≦14.9. It is to be noted that, ascan be seen from a comparison between Examples 1 and 2 to be describedlater, it is possible, by adding a fraction of SrCO₃ as a raw materialafter calcining, to extend the range of b in which high levels of thecoercive force (HcJ) and the residual magnetic flux density (Br) can beattained simultaneously.

The ferrite magnetic material according to the present inventioncomprises the Ca constituent and the Si constituent originatedrespectively from CaCO₃ and SiO₂. These constituents are present mainlyin the grain boundary phase in the ferrite magnetic material, but thestates of these constituents is not clear. As described above, CaCO₃ andSiO₂ have hitherto been added in ferrite magnetic materials for thepurpose of regulating the coercive force (HcJ), the grain size and thelike. However, the present inventors have verified that there can beobtained a ferrite sintered magnet simultaneously having high levels ofcoercive force (HcJ) and residual magnetic flux density (Br) bycontaining CaCO₃ and SiO₂ in a predetermined ratio in the maincomposition represented by the composition formula (1). Thepredetermined ratio (molar ratio) as referred to herein means the ratio,CaCO₃/SiO₂=0.5 to 1.38. The ratio CaCO₃/SiO₂ is preferably 0.6 to 1.1,and more preferably 0.65 to 1.0. The following reason is not clear: thereason why there can be obtained a ferrite sintered magnetsimultaneously having high levels of coercive force (HcJ) and residualmagnetic flux density (Br) by containing CaCO₃ and SiO₂ in apredetermined ratio; however, it may be understood that the followingfact may be involved: the fact that in the case of the ferrite sinteredmagnet, when the ratio CaCO₃/SiO₂ falls within the range specified inthe present invention while the main composition is being the same, themean grain size of the sintered body is made fine.

It is preferable that CaCO₃ and SiO₂ are contained in the followingranges, respectively: CaCO₃: 0.3 to 2.0 wt % and SiO₂: 0.1 to 1.8 wt %.When the amount of CaCO₃ is less than 0.3 wt % and the amount of SiO₂ isless than 0.1 wt %, the effect of addition of CaCO₃ and SiO₂ isinsufficient. When the amount of CaCO₃ exceeds 2.0 wt %, there is a fearof generating a Ca ferrite to provide a factor for degrading themagnetic properties. Also when the amount of SiO₂ exceeds 1.8 wt %, theresidual magnetic flux density (Br) tends to be degraded. The amounts ofCaCO₃ and SiO₂ are preferably set to fall in the following ranges:CaCO₃: 0.5 to 1.1 wt % and SiO₂: 0.3 to 1.3 wt %.

It is to be noted that the Ca constituent may also be added in a formother than CaCO₃, and the Si constituent may also be added in a formother than SiO₂.

The composition of the ferrite magnetic material according to thepresent invention can be measured by means of X-ray fluorescencequantitative analysis or the like. Additionally, the present inventiondoes not exclude the inclusion of elements other than the element(s) A(at least one element selected from Sr, Ba and Pb), Fe, the Caconstituent and the Si constituent. For example, a fraction of the Fe²⁺sites may be replaced with other elements.

The ferrite magnetic material of the present invention, as describedabove, can constitute any of a ferrite sintered magnet, a ferrite magnetpowder, a bonded magnet as a ferrite magnet powder dispersed in a resin,and a magnetic recording medium as a film-like magnetic layer.

The ferrite sintered magnet and the bonded magnet according to thepresent invention are machined to predetermined shapes to be used in awide range of applications as shown below. These can be used as motorsin automobiles for use in fuel pumps, power windows, ABSs (antilockbrake systems), fans, wipers, power steerings, active suspensions,starters, door locks, electric mirrors and the like. These can also beused as motors for use in OA and AV devices such as FDD spindles, VTRcapstans, VTR rotation heads, VTR reels, VTR loading devices, VTR cameracapstans, VTR camera rotation heads, VTR camera zooming devices, VTRcamera focusing devices, capstans in radio cassette players and thelike, spindles for CD, LD and MD, loading in CD, LD and MD, and opticalpickup for CD and LD. These can also be used as motors for use inhousehold electric appliances such as air compressors, refrigeratorcompressors, electric tool drivers, electric fans, fans in microwaveovens, turnplates in microwave ovens, mixer drivers, dryer fans, shaverdrivers, electric toothbrushes and the like. Further, these can be usedas motors for use in FA equipment such as motors for use in robot axes,joint driving devices, robot main axis driving devices, machine tooltable driving devices, machine tool belt driving devices and the like.Among other applications included are suitably applied examples such aselectric generators for use in motorcycles, magnets for use inspeakers/headphones, magnetron tubes, MRI magnetic field generators,CD-ROM clampers, distributor sensors, ABS sensors, fuel/oil levelsensors, magnet latches, isolators and the like.

When a bonded magnet is produced from the ferrite magnet powder of thepresent invention, the mean particle size of the powder is preferablyset to be 0.1 to 5 μm. The mean particle size of the powder for use inthe bonded magnet is more preferably 0.1 to 2 μm, and furthermorepreferably 0.1 to 1 μm.

When a bonded magnet is produced, a ferrite magnet powder is kneadedwith various binders such as a resin, a metal, a rubber and the like,and compacted in a magnetic field or under conditions free from magneticfield. As the binder, NBR rubber, chlorinated polyethylene and polyamideresin are preferable. After compacting, curing is carried out to producea bonded magnet. It is to be noted that a heat treatment to be describedlater is desirably carried out before kneading of the ferrite magnetpowder with a binder.

By using the ferrite magnetic material of the present invention,magnetic recording media each having a magnetic layer can be prepared.This magnetic layer comprises the W-type ferrite phase represented bythe above described composition formula (1). In forming the magneticlayer, for example, an evaporation method and a sputtering method can beused. When the magnetic layer is formed by means of the sputteringmethod, the ferrite sintered magnet according to the present inventionmay be used as a target. Examples of the magnetic recording media mayinclude hard disks, flexible disks and magnetic tapes.

Next, the method for producing a ferrite sintered magnet, among theferrite magnetic materials of the present invention, will be describedbelow. The method for producing the ferrite sintered magnet of thepresent invention comprises a mixing step, a calcining step, apulverizing step, a milling step, a compacting step in a magnetic field,a step for heat treating a compacted body and a sintering step.

Because Fe²⁺ tends to turn into Fe³⁺ in the air, the heat treatmenttemperature, the sintering atmosphere and the like are controlled in themethod for producing a ferrite sintered magnet of the present inventionfor the purpose of controlling Fe²⁺ to be stable. Now, the individualsteps will be described below.

<Mixing Step>

An Fe₂O₃ (hematite) powder is prepared, and a SrCO₃ powder is furtherprepared when Sr is selected as the element A. The SrCO₃ powder and theFe₂O₃ (hematite) powder are weighed out so as for the main compositionto satisfy composition formula (1). For that purpose, more specifically,the amounts of the SrCO₃ powder and the Fe₂O₃ powder are made to fallwithin a range from 1:8.0 to 1:8.6 by molar ratio. In this connection,CaCO₃ and SiO₂ may be added for the purpose of improving the coerciveforce and regulating the grain size. The additive amounts concerned areas above described. In the present invention, powders of Al₂O₃, Cr₂O₃and the like may also be added; the SrCO₃ powder and the Fe₂O₃ powdermay also be added after calcining. After weighing out, these ingredientsare mixed and crushed for 1 to 3 hours with a wet attritor or the like.

An example utilizing a SrCO₃ powder and a Fe₂O₃ powder will be describedbelow; the element A is added as a carbonate in this embodiment, butalternatively may be added as an oxide. Similarly for Fe, Fe may beadded as a compound other than Fe₂O₃ (hematite). Additionally, acompound containing the element A and Fe may also be used.

As for the SrCO₃ powder as the raw material powder for the element A,the total amount thereof may be added in the mixing step, butalternatively, a fraction of the amount thereof may be added aftercalcining. In this way, the improvement of the magnetic properties canbe attained.

In the present invention, a predetermined amount of the Ca constituentand/or a predetermined amount of Si constituent maybe added asadditive(s) in the mixing step, and the addition of the Ca constituentis particularly effective. The Ca constituent may be added, for example,as a CaCO₃ powder or as a CaO powder. The additive amount of the Caconstituent at the time of mixing is set to be 0.01 wt % or more andless than 1.0 wt % in terms of CaCO₃ in relation to the above describedmain constituent comprising the element A and the Fe constituent. Theaddition of the Ca constituent in this range makes the mean grain sizeas fine as equal to or less than 0.6 μm, and furthermore, equal to orless than 0.55 μm, eventually to permit yielding a ferrite magneticmaterial having a coercive force (HcJ) exceeding 3000 Oe.

The additive amount of the Ca constituent is preferably 0.1 to 0.9 wt %,and more preferably 0.2 to 0.8 wt % in terms of CaCO₃.

<Calcining Step>

Subsequently, the mixed powder material obtained in the mixing step iscalcined at 1100 to 1400° C. By conducting this calcining in anonoxidative atmosphere of a gas such as nitrogen gas, argon gas or thelike, the Fe³⁺ in the Fe₂O₃ (hematite) powder is reduced to generateFe²⁺ constituting a W-type ferrite, and thus a W-type ferrite is formed.However, if a sufficient amount of Fe²⁺ can not be ensured at this step,an M phase or a hematite phase are allowed to be present in addition tothe W phase. For the purpose of obtaining a single W-phase ferrite, itis effective to regulate the oxygen partial pressure in calciningbecause when the oxygen partial pressure is decreased, Fe³⁺ tends to beeasily reduced to generate Fe²⁺.

As described above, when the total amount of SrCO₃ is not added beforecalcining, a predetermined amount of the SrCO₃ powder is added aftercalcining.

Also when CaCO₃ and SiO₂ have already been added in the mixing step, itis also possible that the calcined body is milled to a predeterminedgrain size to yield a ferrite magnet powder.

In the present invention, the Ca constituent can be added in the mixingstep, but it is preferable that the Ca constituent is also added afterthe calcining step and before the compacting step. The Ca constituentadded after the calcining step contributes for the purpose of improvingthe coercive force (HcJ) and regulating the grain size. The additiveamount of the Ca constituent at this stage is preferably 0.1 to 2.0 wt %in terms of CaCO₃. When the amount of CaCO₃ exceeds 2.0 wt %, there is afear of generating a Ca ferrite to provide a factor for degrading themagnetic properties. The amount of the Ca constituent added after thecalcining step is preferably 0.2 to 1.5 wt % in terms of CaCO₃, and morepreferably 0.3 to 1.2 wt % in terms of CaCO₃.

The Si constituent, contributing for the purpose of improving thecoercive force (HcJ) and regulating the grain size, may also be added inthe mixing step, but it is preferable that the Si constituent is addedafter the calcining step and before the compacting step in a range from0.2 to 1.4 wt % in terms of SiO₂. When the Si constituent is added in anamount of less than 0.2 wt % in terms of SiO₂, the effect of theaddition of the Si constituent is insufficient, while when the Siconstituent is added in an amount of more than 1.4 wt % in terms ofSiO₂, the residual magnetic flux density (Br) tends to be degraded. Theadditive amount of the Si constituent is preferably 0.2 to 1.0 wt %, andmore preferably 0.3 to 0.8 wt % in terms of SiO₂.

<Pulverizing Step>

The calcined body is generally granular, so that it is preferable todisintegrate the calcined body. In the pulverizing step, a vibrationmill or the like is used to disintegrate the calcined body until themean particle size falls within the range from 0.5 to 10 μm. The powderobtained in this step will be referred to as a coarse powder.

<Milling Step>

In the subsequent milling step, the coarse powder is wet milled or drymilled with an attritor, a ball mill, a jet mill or the like so as forthe particle size to be 1 μm or less, preferably 0.1 to 0.8 μm, and morepreferably 0.1 to 0.6 μm, to yield a fine powder. It is also effectiveto add carbon powder having reduction effect in this step for thepurpose of generating the W-type ferrite in an almost single phase (or asingle phase) state. As described above, CaCO₃ and SiO₂ may be added inadvance of milling for the purpose of improving the coercive force andregulating the grain size.

The milling step is preferably carried out in two separated steps,namely, a first fine milling step and a second fine milling step, or inthree or more steps, from the viewpoint of the magnetic properties. Themilling procedures involved will be described below.

In the first fine milling, the coarse powder is wet milled or dry milledwith an attritor, a ball mill, a jet mill or the like so as for theparticle size to be 1 μm or less, preferably 0.1 to 0.8 μm, and morepreferably 0.1 to 0.6 μm. The first fine milling step is conducted forthe purpose of vanishing the coarse powder, and further for the purposeof making fine the structure after sintering in order to improve themagnetic properties, and accordingly the specific surface area (based onthe BET method) is preferably set to fall within a range from 20 to 25m²/g.

The milling treatment time depends on the milling method adopted; whenthe coarse powder is wet milled with a ball mill, it is recommended thatthe milling treatment is carried out for 60 to 100 hours per 200 g ofthe coarse powder.

For the purpose of improving the coercive force and regulating the grainsize, powders of CaCO₃ and SiO₂, and further, SrCO₃, BaCO₃, Al₂O₃, Cr₂O₃and the like may be added in advance of the first milling.

In a heat treatment step, the fine powder obtained in the fist millingis heat treated by maintaining the fine powder at 600 to 1200° C., morepreferably at 700 to 1000° C., for 1 second to 100 hours.

By passing through the first milling, an ultra fine powder as a powderless than 0.1 μm in particle size is inevitably generated. The presenceof such an ultra fine powder sometimes causes troubles in the subsequentcompacting step. For example, when the proportion of such an ultra finepowder is large, there is caused a trouble in wet compacting such thatno compacting is possible because of the adverse retention of water bythe ultra fine powder. Accordingly, in an embodiment of the presentinvention, a heat treatment (a heat treatment of the powder) is carriedout in advance of the compacting step. More specifically, this heattreatment is carried out for the purpose of reducing the proportion ofthe ultra fine powder by reacting the ultra fine powder less than 0.1 μmin particle size generated in the first milling with the fine powder(for example a fine powder of 0.1 to 0.2 μm in particle size) larger inparticle size than the ultra fine powder. This heat treatment reducesthe proportion of the ultra fine powder, and the compactibility can bethereby improved.

The atmosphere of the heat treatment is recommended to be a nonoxidativeatmosphere similarly to the calcining step. The nonoxidative atmospherein the present invention comprises an atmosphere of an inert gas such asnitrogen gas or Ar gas. The nonoxidative atmosphere of the presentinvention allows inclusion of 10 vol % or less of oxygen. When such anorder of amount of oxygen is included, the oxidation of Fe is negligiblewhen maintained at the above described temperature.

The oxygen amount of the heat treatment atmosphere is preferably 1 vol %or less, and more preferably 0.1 vol % or less.

In the second milling following the heat treatment, the heat-treatedfine powder is wet milled or dry milled with an attritor, a ball mill, ajet mill or the like, to be 1 μm or less in particle size, preferably0.1 to 0.8 μm and more preferably 0.1 to 0.6 μm. The second milling iscarried out for the purpose of regulating the particle size, eliminatingthe necking and improving the dispersibility of an additive oradditives; the specific surface area (based on the BET method) of thesecond fine milled powder is preferably set to fall within a range from10 to 20 m²/g and more preferably from 10 to 15 m²/g. When the specificsurface area is regulated to fall within these ranges, the amount of theultra fine particles is small, if any, and the compactibility is notadversely affected. In other words, by passing through the first millingstep, the step for heat treating the powder and the second milling step,the requirement that the structure after sintering be made fine can besatisfied without adversely affecting the compactibility.

The milling treatment time depends on the milling method adopted; whenthe fine powder is wet milled with a ball mill, it is recommended thatthe milling treatment is carried out for 10 to 40 hours per 200 g of thefine powder. If the second milling step is carried out under theconditions similar to those for the first milling step, ultra finepowder is once again generated, and the desired particle size is almostattained in the first milling step, so that the second milling step isusually alleviated in the milling conditions as compared to the firstmilling step. The judgment as to whether the milling conditions arealleviated or not is recommended to be made on the basis of themechanical energy to be input at the time of milling while notrestricting the focus on the milling time.

For the purpose of obtaining a ferrite magnetic material having highmagnetic properties, it is effective to add powders of CaCO₃ and SiO₂,and further, SrCO₃, BaCO₃ and the like in advance of the second millingstep in order to improve the coercive force (HcJ) and regulate the grainsize.

Carbon powder which displays reduction effect in the sintering step maybe added in advance of the second milling step. The addition of carbonpowder is effective for the purpose of generating the W-type ferrite tobe in an almost single phase (or a single phase) state. The additiveamount of carbon powder (hereinafter referred to as “carbon amount”) isset to fall within a range from 0.05 to 0.7 wt % in relation to the rawmaterial powder. By-constraining the carbon amount within this range,the effect of carbon powder as a reducing agent can be sufficientlyenjoyed in the sintering step to be described later, and a highersaturation magnetization (σs) than without added carbon powder can beobtained. The carbon amount in the present invention is preferably 0.1to 0.65 wt %, and more preferably 0.15 to 0.6 wt %. As the carbon powderto be added, well known substances such as carbon black can be used.

In the present invention, for the purpose of preventing the segregationof the added carbon powder in the compacted body, it is preferable toadd a polyhydric alcohol represented by a general formulaC_(n)(OH)_(n)H_(n+2) in advance of the second milling step. In thisgeneral formula, the number n of carbon atoms is set to be 4 or more.When the number n of carbon atoms is 3 or less, the effect of preventingthe segregation of carbon powder is insufficient. The number n of carbonatoms is preferably 4 to 100, more preferably 4 to 30, furthermorepreferably 4 to 20, and yet furthermore preferably 4 to 12. Sorbitol ispreferable as the polyhydric alcohol, but two or more polyhydricalcohols may be used in combination. In addition to the polyhydricalcohol to be used in the present invention, other dispersants wellknown in the art may further be used.

The above described general formula is a formula referring to a casewhere the skeleton is wholly composed of a chain and does not includeunsaturated bonds. The number of the hydroxy groups and the number ofthe hydrogen atoms in the polyhydric alcohol may be somewhat less thanthose represented by the general formula. In the general formula,unsaturated bonds may be included, without restricting to saturatedbonds. The basic skeleton may be either a chain or a ring, but ispreferably a chain. When the number of the hydroxy groups is 50% or moreof the number n of the carbon atoms, the advantageous effects of thepresent invention is actualized, but it is preferable that the number ofthe hydroxy groups is as larger as possible, and it is most preferablethat the number of the hydroxy groups and the number of the carbon atomsare the same. It is recommended that the additive amount of thepolyhydric alcohol is 0.05 to 5.0 wt %, preferably 0.1 to 3.0 wt %, andmore preferably 0.3 to 2.0 wt % in relation to the powder to be addedwith the polyhydric alcohol. Most of the added polyhydric alcohol isdecomposed to be eliminated in the step for heat treating the compactedbody to be carried out after the compacting step in a magnetic field.The remaining polyhydric alcohol which has not been decomposed to beeliminated in the step for heat treating the compacted body isdecomposed to be eliminated in the subsequent sintering step.

<Compacting Step in a Magnetic Field>

The fine powder obtained in the above described milling steps issubjected to wet or dry compacting in a magnetic field. It is preferableto carry out wet compacting for the purpose of enhancing the orientationdegree, and accordingly description will be made below on the case wherewet compacting is carried out.

When wet compacting is adopted, the second milling step is carried outin a wet manner, and the slurry after wet milling is concentrated toprepare a slurry for wet compacting. The concentration may be carriedout by means of centrifugal separation, a filter press, or the like. Inthis case, it is preferable that the proportion of the ferrite magnetpowder amounts to 30 to 80 wt % of the slurry for wet compacting. Whenthe water is used as a dispersion medium, it is also preferable to addsurfactants such as gluconic acid (salt) and sorbitol. Then, compactingin a magnetic field is carried out by use of the slurry for wetcompacting. It is recommended that the compacting pressure is set to beof the order of 0.1 to 0.5 ton/cm², the applied magnetic field is set tobe of the order of 5 to 15 kOe. The dispersion medium is not limited towater, but may be a nonaqueous medium. When a nonaqueous dispersionmedium is used, an organic solvent such as toluene or xylene may beused. When toluene or xylene is used as a nonaqueous dispersion medium,it is preferable to add a surfactant such as oleic acid.

<Step for Heat Treating a Compacted Body>

In the present step, the compacted body is subjected to a heat treatmentin which the compacted body is maintained at temperatures as low as 100to 450° C., and more preferably as low as 200 to 350° C., for 1 to 4hours. By carrying out this heat treatment in the air, a fraction of Fe2is oxidized into Fe³⁺. In other words, in the present step, by makingthe reaction from Fe²⁺ to Fe³⁺ proceed to a certain extent, the amountof Fe²⁺ is controlled to a predetermined value. In the present step, thedispersion medium is eliminated.

<Sintering Step>

In the following sintering step, the compacted body is sintered at 1100to 1270° C., and more preferably 1160 to 1240° C. for 0.5 to 3 hours.The sintering atmosphere should be a nonoxidative atmosphere on the samegrounds as those for the calcining step. In the present step, the carbonpowder added before the second milling step is eliminated.

By passing through the above described steps, the ferrite sinteredmagnet of the present invention can be obtained. According to theferrite sintered magnet of the present invention, a residual magneticflux density (Br) of 4.5 kG or more and a coercive force (HcJ) of 3 kOeor more can be simultaneously attained. In the present invention, theobtained sintered magnet can be milled to be used as a ferrite magnetpowder. The ferrite magnet powder can be used for bonded magnets.

In the above, description has been made on the method for producing theferrite sintered magnet; also when the ferrite magnet powder isproduced, the same steps can be appropriately adopted. The ferritemagnet powder according to the present invention may be produced throughtwo processes in which it is produced from the calcined body and fromthe sintered body, respectively.

When produced from the calcined body, CaCO₃ and SiO₂ are added beforethe calcining step. The calcined body having been obtained by addingCaCO₃ and SiO₂ is subjected to pulverizing and milling to yield theferrite magnet powder. The ferrite magnet powder thus obtained issubjected to the above described heat treatment, and then put intopractical use as the ferrite magnet powder. For example, bonded magnetsare produced by using the ferrite magnet powder having been subjected toheat treatment. The ferrite magnet powder is not only used for bondedmagnets, but can be used for producing ferrite sintered magnets.Accordingly, the ferrite magnet powder of the present invention may alsobe produced within the steps for producing the ferrite sintered magnet.The particle size of the ferrite magnet powder may be different whenused for bonded magnets from when used for ferrite sintered magnets, asthe case may be.

When the ferrite magnet powder is produced from the ferrite sinteredmagnet, CaCO₃ and SiO₂ may be added at any step before the sinteringstep. The ferrite magnet powder of the present invention can be producedby appropriately milling the ferrite sintered magnet obtained on thebasis of the above described steps.

As described above, the ferrite magnet powder of the present inventioncomprises a form of a calcined powder, a form of a powder milled afterundergoing calcining and sintering, and a form of a powder heat-treatedafter undergoing milling subsequently to calcining.

As described above, description has been made on the method forproducing the ferrite magnetic material of the present invention, andthis production method is outlined in a flowchart in FIG. 1. In FIG. 1,the steps surrounded with a solid line are the steps indispensable forthe production of the sintered magnet and the steps surrounded with adotted line are optional steps. For example, addition of SrCO₃ isindispensable for the mixing (1), but is optional in any one of (2) to(4).

Hereinafter, examples of the present invention will be described.

EXAMPLE 1

A ferrite sintered magnet was prepared according to the followingprocedures.

As raw material powders, a Fe₂O₃ powder (primary particle size: 0.3 μm)and a SrCO₃ powder (primary particle size: 2 μm) were prepared. Theseraw material powders were weighed out so as for a+b in the above formula(1) to be the mixing compositions shown in FIG. 2. After weighing out,the powders each having one of the compositions shown in FIG. 2 weremixed and crushed with a wet attritor for 2 hours.

Then, each of the mixed powders was dried and sized, and thereaftercalcined in nitrogen at 1300° C. for 1 hour to yield a powdery calcinedbody. The calcined body was pulverized with a dry vibration mill for 10minutes to yield a coarse powder of 1 μm in mean particle size.

Subsequently, the coarse powder was milled. The milling was carried outwith a ball mill in two steps. In the first milling, 210 g of the coarsepowder was added with 400 ml of water, and the mixture thus obtained wasmilled for 88 hours. After the first milling, the fine powder thusobtained was subjected to a heat treatment under the conditions that thefine powder was maintained in an atmosphere of N₂ gas at 800° C. for 1hour. The rate of the temperature increase up to 800° C. and the rate ofthe temperature decrease from the 800° C. were set at 5° C./min.Subsequently, the second milling in which wet milling was carried outwith a ball mill for 25 hours was carried out to yield a slurry for wetcompacting. It is to be noted that before the second milling, a SiO₂powder (primary particle size: 0.01 μm) and a CaCO₃ powder (primaryparticle size: 1 μm) were added in the amounts shown in FIG. 2, andfurther a carbon powder (primary particle size: 0.05 μm) was added in anamount of 0.3 wt %, and sorbitol (primary particle size: 10 μm) as apolyhydric alcohol was added in an amount of 1.2 wt %. The amount of thecalcined powder in the slurry was 33 wt %. Then, the slurry aftercompletion of milling was concentrated with a centrifugal separator toyield the slurry for wet compacting, which was used to performcompacting in a magnetic field. The applied magnetic field (a verticalmagnetic field) was 12 kOe (1000 kA/m), and each of the obtainedcompacted bodies had a cylindrical form of 30 mm in diameter and 15 mmin height.

Each of the compacted bodies obtained as described above was subjectedto a heat treatment in which the compacted body was maintained at 225°C. for 3 hours in the air, and thereafter was sintered in nitrogen witha temperature increase rate of 5° C./min and at a maximum temperature of1200° C. for 1 hour to yield a sintered body. The composition of each ofthe sintered bodies obtained as described above was measured with anX-ray fluorescence spectrometer for quantitative analysis “SIMULTIX3550” manufactured by Rigaku Corp., and the values of a and b in theabove formula (1) were derived. The coercive force (HcJ) and theresidual magnetic flux density (Br) were measured for each of theobtained sintered bodies. The results thus obtained are also shown inFIG. 2. It is to be noted that the coercive force (HcJ) and the residualmagnetic flux density (Br) of each of the obtained sintered bodies wereevaluated in such a way that the upper and lower surfaces of thesintered body were machined and thereafter a B-H tracer was used with amaximum applied magnetic field of 25 kOe.

As shown in FIG. 2, in each of the cases where CaCO₃/SiO₂ was 1.40 or0.47, no coercive force (HcJ) of 3 kOe or more and no residual magneticflux density (Br) of 4.5 kG or more were obtained. On the contrary, inthe cases where CaCO₃/SiO₂ was 0.93 or 0.70, a coercive force (HcJ) of 3kOe or more and a residual magnetic flux density (Br) of 4.5 kG or morewere able to be obtained for the b value of 14.6 or 14.8.

As described above, when CaCO₃ and SiO₂ were added, by specifyingCaCO₃/SiO₂ and a and b in the above composition formula (1), thecoercive force (HcJ) and the residual magnetic flux density (Br) wereable to be made to attain simultaneously high levels.

The constituent phase of each of the obtained sintered bodies wereobserved by X-ray diffraction, and was found to be a single phasecomposed of the W phase (“W” in FIG. 2), except for a sintered bodywhich also contained the M phase (“W+M” in FIG. 2), with a molar ratioof the M phase portion being of the order of 20%. The conditions for theX-ray diffraction were as follows:

X-ray generator: 3 kW; X-ray tube voltage: 45 kV; X-ray tube current: 40mA; Sampling width: 0.02 deg; Scanning speed: 4.00 deg/min; Divergingslit: 1.00 deg; Scattering slit: 1.00 deg; Receiving slit: 0.30 mm.

Mean grain sizes were measured for some of the sintered bodies shown inFIG. 2 with a=2.0 and b=14.8 in the above composition formula (1). Theresults obtained are shown in FIG. 3. As shown in FIG. 3, CaCO₃/SiO₂ andthe mean grain size are interrelated with each other; as can be seen,the smaller was CaCO₃/SiO₂, the smaller was the mean grain size. Whenthe CaCO₃/SiO₂ of the present invention fell within the range of thepresent invention, it was possible to make the grain be as fine as 0.8μm or less in mean grain size. It is to be noted that the measurement ofthe mean grain size was carried out as follows: The A surface (thesurface containing the a-axis and the c-axis) of a sintered body waspolished, thereafter subjected to acid etching, then the SEM (scanningelectron microscope) microgram of the surface was taken; the individualgrains were identified in the microgram, and the maximum diameterpassing through the center of gravity of each of the individual grainswas derived on the basis of image analysis to be taken as a grain sizeof the sintered body; and the mean grain size was obtained in such a waythat the grain sizes of about 100 grains per a sample were measured andall the grain sizes thus obtained were averaged.

EXAMPLE 2

As raw material powders, a Fe₂O₃ powder (primary particle size: 0.3 μm)and a SrCO₃ powder (primary particle size: 2 μm) were prepared. Theseraw material powders were weighed out so as for a+b to be the mixingcompositions shown in FIG. 4. After weighing out, the powders were mixedand milled with a wet attritor for 2 hours.

Then, each of the thus obtained mixtures was calcined in nitrogen at1300° C. for 1 hour to yield a powdery calcined body. The calcined bodywas milled with a dry vibration mill for 10 minutes to yield a coarsepowder of 1 μm in mean particle size.

Subsequently, milling was carried out. The milling was carried out witha ball mill in two steps. In the first milling, 210 g of the coarsepowder was added with 400 ml of water, and the mixture thus obtained wasmilled for 88 hours. After the first milling, the fine powder thusobtained was subjected to a heat treatment under the conditions that thefine powder was maintained in an atmosphere of N₂ gas at 800° C. for 1hour. The rate of the temperature increase up to the heating andmaintaining temperature and the rate of the temperature decrease fromthe heating and maintaining temperature were set at 5° C./min.Subsequently, the second milling in which wet milling was carried outwith a ball mill for 25 hours was carried out to yield a slurry for wetcompacting. It is to be noted that before the start of the secondmilling, a SrCO₃ powder (primary particle size: 2 μm), a SiO₂ powder(primary particle size: 0.01 μm) and a CaCO₃ powder (primary particlesize: 1 μm) were added in the amounts shown in FIG. 4, and further acarbon powder (primary particle size: 0.05 μm) was added in an amount of0.3 wt %, and sorbitol (primary particle size: 10 μm) as a polyhydricalcohol was added in an amount of 1.2 wt %. The amount of the calcinedpowder in the slurry was 33 wt %. Then, the slurry after completion ofmilling was concentrated with a centrifugal separator to yield theslurry for wet compacting, which was used to perform compacting in amagnetic field. The applied magnetic field (a vertical magnetic field)was 12 kOe (1000 kA/m), and each of the obtained compacted bodies was asolid cylinder of 30 mm in diameter and 15 mm in height.

Each of the compacted bodies obtained as described above was subjectedto a heat treatment in which the compacted body was maintained at 225°C. for 3 hours in the air, and thereafter was sintered in nitrogen witha temperature increase rate of 5° C./min and at a maximum temperature of1200° C. for 1 hour to yield a sintered body. The composition of each ofthe sintered bodies obtained as described above was measured with anX-ray fluorescence spectrometer for quantitative analysis “SIMULTIX3550” manufactured by Rigaku Corp., and the values of a and b in theabove formula (1) were derived. The coercive force (HcJ) and theresidual magnetic flux density (Br) were measured for each of theobtained sintered bodies. The results thus obtained are also shown inFIG. 4. It is to be noted that the coercive force (HcJ) and the residualmagnetic flux density (Br) of each of the obtained sintered bodies wereevaluated in such a way that the upper and lower surfaces of thesintered body were machined and thereafter a B-H tracer was used with amaximum applied magnetic field of 25 kOe.

As shown in FIG. 4, the coercive force (HcJ) and the residual magneticflux density (Br) were improved by adding the SrCO₃ powder as a rawmaterial after calcining, more specifically, before the start of thesecond milling. In particular, even in the range of b where no coerciveforce (HcJ) of 3.0 kOe or more and no residual magnetic flux density(Br) of 4.5 kG or more were able to be obtained in Example 1, coerciveforces (HcJ) of 3.0 kOe or more and residual magnetic flux densities(Br) of 4.5 kG or more were able to be obtained. In other words, byadding after calcining a fraction of the SrCO₃ powder, the range of b inthe above composition formula (1) where a coercive force (HcJ) of 3.0kOe or more and a residual magnetic flux density (Br) of 4.5 kG or morewere able to be simultaneously attained was able to be extended. Morespecifically, in Example 1, a coercive force (HcJ) of 3.0 kOe or moreand a residual magnetic flux density (Br) of 4.5 kG or more were able tobe simultaneously attained only in the cases where b=14.6 or 14.8; onthe contrary, by adding after calcining a fraction of the SrCO₃ powder,coercive forces (HcJ) of 3.0 kOe or more and residual magnetic fluxdensities (Br) of 4.5 kG or more were able to be simultaneously attainedin the range of 12.3≦b≦15.4.

EXAMPLE 3

As raw material powders, a Fe₂O₃ powder (primary particle size: 0.3 μm)and a SrCO₃ powder (primary particle size: 2 μm) were prepared. Theseraw material powders were weighed out so as for a+b to be the mixingcompositions shown in FIG. 5. After weighing out, these powders weremixed and milled with a wet attritor for 2 hours.

Then, each of the thus obtained mixtures was calcined in nitrogen at1300° C. for 1 hour to yield a powdery calcined body. The calcined bodywas milled with a dry vibration mill for 10 minutes to yield a coarsepowder of 1 μm in mean particle size.

Subsequently, milling was carried out. The milling was carried out witha ball mill in two steps. In the first milling, 210 g of the coarsepowder was added with 400 ml of water, and the mixture thus obtained wasmilled for 88 hours. After the first milling, the fine powder thusobtained was subjected to a heat treatment under the conditions that thefine powder was maintained in an atmosphere of N₂ gas at 800° C. for 1hour. The rate of the temperature increase up to the heating andmaintaining temperature and the rate of the temperature decrease fromthe heating and maintaining temperature were set at 5° C./min.Subsequently, the second milling in which wet milling was carried outwith a ball mill for 25 hours was carried out to yield a slurry for wetcompacting. It is to be noted that before the start of the secondmilling, a SrCO₃ powder (primary particle size: 2 μm), a SiO₂ powder(primary particle size: 0.01 μm) and a CaCO₃ powder (primary particlesize: 1 μm) were added in the amounts shown in FIG. 5, and further acarbon powder (primary particle size: 0.05 μm) was added in an amount of0.3 wt %, and sorbitol (primary particle size: 10 μm) as a polyhydricalcohol was added in an amount of 1.2 wt %. The amount of the calcinedpowder in the slurry was 33 wt %. Then, the slurry after completion ofmilling was concentrated with a centrifugal separator to yield theslurry for wet compacting, which was used to perform compacting in amagnetic field. The applied magnetic field (a vertical magnetic field)was 12 kOe (1000 kA/m), and each of the obtained compacted bodies was asolid cylinder of 30 mm in diameter and 15 mm in height.

Each of the compacted bodies obtained as described above was subjectedto a heat treatment in which the compacted body was maintained at atemperature shown in FIG. 5 for 3 hours in the air, and thereafter wassintered in nitrogen with a temperature increase rate of 5° C./min andat a maximum temperature of 1200° C. for 1 hour to yield a sinteredbody. The composition of each of the sintered bodies obtained asdescribed above was measured with an X-ray fluorescence spectrometer forquantitative analysis “SIMULTIX 3550” manufactured by Rigaku Corp., andthe values of a and b in the above formula (1) were derived. Themagnetic properties of each of the obtained sintered bodies wereevaluated in such a way that the upper and lower surfaces of thesintered body were machined and thereafter a B-H tracer was used with amaximum applied magnetic field of 25 kOe. The results thus obtained arealso shown in FIG. 5.

As shown in FIG. 5, by controlling the conditions involving the amountof SrCO₃ added before the second milling, the heat treatment temperaturefor the compacted body and the like, a coercive force (HcJ) of 3.0 kOeor more and a residual magnetic flux density (Br) of 4.5 kG or more wereable to be simultaneously attained over a wide range of a of the aboveformula (1).

EXAMPLE 4

As raw material powders, a Fe₂O₃ powder (primary particle size: 0.3 μm)and a SrCO₃ powder (primary particle size: 2 μm) were prepared. Theseraw material powders to constitute the main constituent were weighed outso as to give the mixing compositions shown in FIG. 6, and thereafter aCaCO₃ powder (primary particle size: 1 μm) was added in an amount of 0to 1.0 wt % in relation to the raw material powders constituting themain constituent. Then, the powder mixtures thus obtained were mixed andmilled with a wet attritor for 2 hours.

Subsequently, calcining was carried out. A tube furnace was used forcalcining, and the calcining was carried out under the conditions thatthe powder mixtures were maintained in an atmosphere of N₂ gas for 1hour. The heating and maintaining temperature was set at 1300° C. Therate of the temperature increase up to the heating and maintainingtemperature and the rate of the temperature decrease from the heatingand maintaining temperature were set at 5° C./min.

Then, pulverizing was carried out with a vibration mill. In thepulverization with a vibration mill, 220 g of a calcined body was milledfor 10 minutes.

The following milling was carried out with a ball mill in two steps. Inthe first milling, 210 g of a coarse milled powder was added with 400 mlof water and the mixture thus obtained was milled for 88 hours.

After the first milling, the fine milled powder thus obtained wassubjected to a heat treatment under the conditions that the fine milledpowder was maintained in an atmosphere of N₂ gas at 800° C. for 10minutes or for 1 hour. The rate of the temperature increase up to theheating and maintaining temperature and the rate of the temperaturedecrease from the heating and maintaining temperature were set at 5°C./min.

Subsequently, the second milling in which wet milling was carried outwith a ball mill was carried out to yield a slurry for wet compacting.It is to be noted that before the second milling, a BaCO₃ powder(primary particle size: 0.05 μm) in an amount of 1.75 wt %, a CaCO₃powder (primary particle size: 1 μm) in an amount of 0.7 wt %, a SiO₂powder (primary particle size: 0.01 μm) in an amount of 0.6 wt % and acarbon powder (primary particle size: 0.05 μm) in an amount of 0.4 wt %were added, and further sorbitol (primary particle size: 10 μm) as apolyhydric alcohol was added in an amount of 1.2 wt % in relation toeach of the fine milled powder subjected to the above described heattreatment.

The slurries obtained by applying the second milling were concentratedwith a centrifugal separator, and the thus concentrated slurries for wetcompacting were used to perform compacting in a magnetic field. Theapplied magnetic field (a vertical magnetic field) was 12 kOe (1000kA/m), and each of the obtained compacted bodies was a solid cylinder of30 mm in diameter and 15 mm in height. No failure was caused in any runof compacting. Each of the compacted bodies thus obtained was heattreated in the air at 300° C. for 3 hours, and then sintered innitrogen, with a temperature increase rate of 5° C./min, at a maximumtemperature of 1190° C. for 1 hour to yield a sintered body. Thecomposition of each of the sintered bodies obtained as described abovewas measured with an X-ray fluorescence spectrometer for quantitativeanalysis “SIMULTIX 3550” manufactured by Rigaku Corp., and the values ofa and bin the above formula (1) were derived. The coercive force (HcJ),the residual magnetic flux density (Br) and the squareness (Hk/HcJ) weremeasured for each of the obtained sintered bodies. The results thusobtained are shown in FIG. 6. It is to be noted that the coercive force(HcJ) and the residual magnetic flux density (Br) of each of theobtained sintered bodies were measured in such a way that the upper andlower surfaces of the sintered body were machined and thereafter a B-Htracer was used with a maximum applied magnetic field of 25 kOe. Here,Hk represents an external magnetic field strength at which the magneticflux density amounts to 90% of the residual magnetic flux density (Br)in the the second quadrant of magnetic hysteresis loop. When Hk is low,no high maximum energy product can be obtained. The squareness Hk/HcJmakes an index representing the performances of a magnet and exhibits adegree of squareness in the second quadrant of the magnetic hysteresisloop.

The relation between the additive amount of the Ca constituent at thetime of mixing and the coercive force (HcJ) is shown in FIG. 7, and therelation between the additive amount of the Ca constituent at the timeof mixing and the residual magnetic flux density (Br) is shown in FIG.8.

As shown in FIGS. 6 to 8, addition of the Ca constituent at the time ofmixing improved the coercive forces (HcJ) and the residual magnetic fluxdensities (Br) as compared to the cases where no Ca constituent wasadded. When the additive amount of the Ca constituent at the time ofmixing reached 1 wt %, the coercive force (HcJ) was made lower thanthose obtained without the Ca constituent. From the above results, inthe present invention, the additive amount of the Ca constituent at thetime of mixing is set to be less than 1 wt %, and preferably 0.01 to 0.9wt % in terms of CaCO₃.

As shown in FIGS. 6 to 8, according to the samples in which the Caconstituent was added within the range recommended by the presentinvention at the time of mixing, the coercive forces (HcJ) of 3200 Oe ormore, the residual magnetic flux densities of 4700 G or more and thesquareness (Hk/HcJ) values of 90% or more were able to be obtained.

EXAMPLE 5

Sintered bodies were prepared under the same conditions as in Example 4except that, as a raw material powder constituting the main constituent,a BaCO₃ powder (primary particle size: 0.05 μm) was further prepared,weighing out was carried out to give the mixing compositions shown inFIG. 6, and thereafter, a CaCO₃ powder (primary particle size: 1 μm) wasadded in an amount of 0 to 1.33 wt % in relation to the raw materialpowders constituting the main constituent; and additionally, at the timeof the second milling, there were added a SrCO₃ powder (primary particlesize: 2 μm) in an amount of 0.7 wt %, a BaCO₃ powder (primary particlesize: 0.05 μm) in an amount of 1.4 wt %, a CaCO₃ powder (primaryparticle size: 1 μm) in an amount of 0.35 wt %, a SiO₂ powder (primaryparticle size: 0.01 μm) in an amount of 0.6 wt %, a carbon powder(primary particle size: 0.05 μm) in an amount of 0.4 wt % and sorbitol(primary particle size: 10 μm) in an amount of 1.2 wt %. It is to benoted that the ratios between Sr and Ba in the obtained sintered bodieswere as follows:

The additive amount of the Ca constituent at the time of mixing=0;Sr:Ba=0.66:0.34

The additive amount of the Ca constituent at the time of mixing=0.33 wt%; Sr:Ba=0.64:0.36

The additive amount of the Ca constituent at the time of mixing=0.67 wt%; Sr:Ba=0.63:0.37

The additive amount of the Ca constituent at the time of mixing=1.33 wt%; Sr:Ba=0.58:0.42

For each of the sintered bodies obtained in the same manner as inExample 4, the composition, the coercive force (HcJ), the residualmagnetic flux density (Br) and the squareness (Hk/HcJ) were measured.The measurement results thus obtained are also shown in FIG. 6.

As shown in FIG. 6, also when Sr and Ba were added in combination at thetime of mixing, the tendencies similar to those in Example 4 were ableto be verified.

In FIG. 6, from a comparison of the samples in which Sr and Ba wereselected as the elements A constituting the main constituent with thesamples in which only Sr was selected as the element A, it has beenfound that the former samples showed higher coercive forces (HcJ).Consequently, it can be said that the combined addition of Sr and Ba toconstitute the main constituent furthermore improves the coercive force(HcJ) than the single addition of Sr to constitute the main constituent.

The samples in which the additive amount of the Ca constituent at thetime of mixing was 0.33 wt % or 0.67 wt % attained the coercive forces(HcJ) of 3400 Oe or more, or 3500 Oe or more while having the residualmagnetic flux densities (Br) of 4700 G or more. It is inferred thatthese high magnetic properties are also originated from the addition ofnot only the Ba constituent but the Sr constituent in combination as theadditives at the time of the second milling.

Next, the mean grain size was measured for each of the samples preparedin Example 5. It is to be noted that the measurement of the mean grainsize was carried out as follows: The A surface (the surface containingthe a-axis and the c-axis) of a sintered body was polished, thereaftersubjected to acid etching, then the SEM (scanning electron microscope)microgram of the surface was taken; the individual grains wereidentified in the microgram, and the maximum diameter passing throughthe center of gravity of each of the grains was derived on the basis ofimage analysis to be taken as a grain size of the sintered body; and themean grain size was obtained in such a way that the grain sizes of about100 grains per a sample were measured and all the grain sizes thusobtained were averaged.

The relation between the additive amount of the Ca constituent at thetime of mixing and the mean grain size is shown in FIG. 9, and therelation between the additive amount of the Ca constituent at the timeof mixing and the coercive force (HcJ) is shown in FIG. 10.

As shown in FIG. 9, it can be seen that the addition of a certainpredetermined amount of the Ca constituent made the grains fine. Thetendency in the effect of making the grains fine is consistent with thetendency in the effect of improving the coercive force (HcJ) shown inFIG. 10, and consequently it is conceivable that the coercive force(HcJ) improvement effect is originated from making the grains fine.

EXAMPLE 6

Three types of sintered bodies were prepared under the same conditionsas in Example 5 except that after weighing out was carried out so as togive the mixing compositions shown in FIG. 6, and thereafter a CaCO₃powder (primary particle size: 1 μm) was added in an amount of 0 to 1.0wt % in relation to the raw material powders constituting the mainconstituent. In each of the obtained sintered bodies, the ratio betweenSr and Ba was such that Sr:Ba=0.63:0.37. Although in Examples 4 and 5described above, the amount of the Sr constituent was decreasedaccording to the additive amount of the Ca constituent at the time ofmixing, such an operation was not carried out in Example 6.

The composition, the coercive force (HcJ), the residual magnetic fluxdensity (Br) and the squareness (Hk/HcJ) were measured for each of theobtained sintered bodies in the same manner as in Example 4. The resultsthus obtained are shown in FIG. 6.

As shown in FIG. 6, even when the operation of reducing the amount ofthe Sr constituent according to the additive amount of the Caconstituent at the time of mixing was not carried out, tendenciessimilar to those in Examples 4 and 5 have been verified.

EXAMPLE 7

Sintered bodies were prepared under the same conditions as in Example 4except that after weighing out was carried out so as to give the mixingcompositions shown in FIG. 6, a SiO₂ powder (primary particle size: 0.01μm) in an amount of 0 or 0.6 wt % and a CaCO₃ powder (primary particlesize: 1 μm) in an amount of 0, 0.33 or 0.68 wt % were added in relationto the raw material powders constituting the main constituent; andadditionally, at the time of the second milling, there were added aSrCO₃ powder (primary particle size: 2 μm) in an amount of 0.7 wt %, aBaCO₃ powder (primary particle size: 0.05 μm) in an amount of 1.4 wt %,a CaCO₃ powder (primary particle size: 1 μm) in an amount of 0.35 wt %,a SiO₂ powder (primary particle size: 0.01 μm) in an amount of 0 or 0.6wt %, a carbon powder (primary particle size: 0.05 μm) in an amount of0.4 wt % and sorbitol (primary particle size: 10 μm) in an amount of 1.2wt %.

The composition, the coercive force (HcJ), the residual magnetic fluxdensity (Br) and the squareness (Hk/HcJ) were measured for each of theobtained sintered bodies in the same manner as in Example 4. The resultsthus obtained are shown in FIG. 6.

As shown in FIG. 6, it can be seen that even when the Si constituent wasadded at the time of mixing concomitantly with the Ca constituent, highmagnetic properties were exhibited.

INDUSTRIAL APPLICABILITY

The present invention can provide a ferrite magnetic material capable ofmaking the coercive force (HcJ) and the residual magnetic flux density(Br) simultaneously attain high levels, in particular, such a materialhaving a W-type ferrite as the main phase thereof, by adopting anoptimal composition also in consideration of additives, and further byelaborating the method for producing the material.

1. A ferrite magnetic material characterized by comprising as a mainconstituent a compound represented by a composition formula,AFe2+aFe3+bO27 (wherein 1.1≦a≦2.4, 12.3≦b≦16.1; and A comprises at leastone element selected from Sr, Ba and Pb), and as additives a Caconstituent in terms of CaCO3 and a Si constituent in terms of SiO2 soas to satisfy the relation CaCO3/SiO2=0.5 to 1.38 (molar ratio).
 2. Theferrite magnetic material according to claim 1, characterized in thatsaid a satisfies the relation, 1.5≦a≦2.4.
 3. The ferrite magneticmaterial according to claim 1, characterized in that said b satisfiesthe relation, 12.9≦b≦15.6.
 4. The ferrite magnetic material according toclaim 1, characterized in that CaCO3/SiO_(2=0.6) to 1.1 (molar ratio).5. The ferrite magnetic material according to claim 1, characterized inthat said ferrite magnetic material constitutes any of a ferritesintered magnet, a ferrite magnet powder, a bonded magnet in which saidferrite magnetic material is dispersed as a ferrite magnet powder in aresin, and a magnetic recording medium in which said ferrite magneticmaterial is contained as a film-like magnetic phase.
 6. The ferritemagnetic material according to claim 5, characterized in that saidferrite sintered magnet is 0.8 μm or less in mean grain size.
 7. Theferrite magnetic material according to claim 1, characterized in thatsaid ferrite magnetic material comprises a hexagonal W-type ferrite as amain phase.
 8. A method for producing a hexagonal W-type ferritemagnetic material characterized by comprising the steps of: (a)obtaining a raw material powder comprising A (wherein A comprises atleast one element selected from Sr, Ba and Pb) and Fe; (b) obtaining acalcined body by maintaining said raw material powder at a predeterminedtemperature for a predetermined time; and (c) milling said calcinedbody, wherein CaCO3 and/or SiO2 are added before and/or after said step(b) so that the hexagonal W-type ferrite magnetic material comprises aCa constituent in terms of CaCO3 and a Si constituent in terms of SiO2so as to satisfy the relation of molar ratio CaCO3/SiO2=0.5 to 1.38. 9.The method for producing a hexagonal W-type ferrite magnetic materialaccording to claim 8, characterized by further comprising a step (d) forsintering the milled powder obtained in said step (c).
 10. The methodfor producing a hexagonal W-type ferrite magnetic material according toclaim 8, characterized by preparing a ferrite magnet powder bypulverizing the sintered body obtained in said step (d).
 11. The methodfor producing a hexagonal W-type ferrite magnetic material according toclaim 8, characterized by preparing a ferrite magnet powder by said step(c).
 12. The method for producing a hexagonal W-type ferrite magneticmaterial according to claim 8, characterized by adding a fraction ofsaid A after calcining.
 13. The method for producing a hexagonal W-typeferrite magnetic material according to claim 8, characterized in thatsaid Ca constituent is added in said step (a) in an amount of 0.01 wt %or more and less than 1.0 wt % in terms of CaCO3 in relation to said rawmaterial powder.
 14. The method for producing a hexagonal W-type ferritemagnetic material according to claim 9, characterized in that: said Caconstituent is added in said step (a) in an amount of 0.01 wt % or moreand less than 1.0 wt % in terms of CaCO3 in relation to said rawmaterial powder; and said Ca constituent is further added in an amountof 0.1 to 2.0 wt % in terms of CaCO3 after said step (b) and before saidsintering step (d).